The present disclosure relates to image forming apparatuses such as copiers, printers, facsimile machines, and so on, and in particular, relates to image forming apparatuses including a developing device having a first conveyance chamber and a second conveyance chamber disposed above the first conveyance chamber.
In an image forming apparatus, an electrostatic latent image formed on an image carrier, such as a photosensitive member, is developed by a developing device and visualized as a toner image. A known example of such a developing device is one that includes a developing container which holds therein a developer, first and second stirring-conveyance members which convey the developer, while stirring the developer, and a developing roller (a developer carrier) which carries thereon the developer supplied thereto from the second stirring-conveyance member. The first stirring-conveyance member conveys the developer to one side in the axial direction of the developing roller, and the second stirring-conveyance member supplies the developer to the developing roller while conveying the developer to the other side (the side opposite from the one side).
In recent years, there has been an increasing demand for miniaturization of image forming apparatuses, and in particular, in color image forming apparatuses, in which a plurality of developing devices are disposed, there has been a demand for miniaturization of developing devices. As an example of developing devices meeting such a demand, there is known one that includes a first conveyance chamber inside which a first stirring-conveyance member is disposed and a second conveyance chamber which is disposed above the first conveyance chamber and inside which a second stirring-conveyance member is disposed. By arranging the first conveyance chamber and the second conveyance chamber one above the other in this developing device, it is possible to make it compact in the horizontal direction. Accordingly, it is possible to reduce space for installing the developing device, and thus, it is possible to achieve the miniaturization of image forming apparatuses.
However, in the above-described developing device, there is an area where the developer is conveyed against gravity, from the first conveyance chamber to the second conveyance chamber. With this structure, when change in flowability of the developer results from factors such as durable printing and environmental variation, the circulation balance of the developer is likely to change, and thus uneven distribution of the developer is likely to occur inside the developing device. When, in such a condition, toner is replenished to a portion where only a small amount of developer exists, it will create a portion where the concentration of the replenished toner is locally high. The replenished toner is not sufficiently mixed with a carrier in the developer and thus is low in charge amount, and accordingly, when the portion with a high replenished-toner concentration is used for development, it will result in problems such as fogged images and uneven image density.
To prevent such problems, there have been proposed various methods for fully mixing developer inside a developing device with replenished toner. For example, there has been known a developing device which has a communication path through which the developer is delivered from a developing chamber to a stirring chamber by conveying the developer to fall from a downstream side of the developing chamber to an upstream side of the stirring chamber in the developer conveyance direction, and in which replenished developer replenished through a developer replenishing port above the stirring chamber falls onto an area in the stirring chamber where the communication path joins the stirring chamber.
Furthermore, there has been known a developing device where, for the purpose of maintaining the circulation balance, after executing a low speed mode in which a developer carrier and a conveyance member are driven at a speed lower than usual, an idle driving mode is executed in which the conveyance member is idly driven at a high driving speed for a predetermined time.